The inspection of bridges for tne purpose of determining physical condition and subsequent corrective action to eliminate unsafe situations is a large and complex task. In the United States, there are over a half million highway bridges requiring large numbers of personnel to gather and evaluate data concerning various factors influencing the physical integrity of these bridges. One of the more important factors is the fatigue condition of the structural members of the bridge and the components which fasten together the structural members. As the bridge is exposed time varying loads caused by the passage of vehicles over the bridge, the structural members and fastener components may crack or rupture or be subjected to fatigue damage by the alternating application and relaxation of stresses caused by these loads. The net effect on these parts is dependent upon the magnitude and frequency of the applied loads.
Various solutions to the problems introduced by fatigue have been put into use in the past. One approach to the fatigue problem has been to routinely replace critical parts without regard as to whether or not these parts actually have been damaged. This is quite wasteful and expensive. Moreover, if the replacement is limited to selected critical parts, the fatigue condition of other parts is ignored.
Another approach has employed non-destructive testing methods to determine the condition of bridge parts. Specialized instrumentation, based for example on materials evaluation by means of x-rays, ultrasonics, stereophotogrammetry or other tests, can be used to examine structural members and fastener components on a scheduled basis to determine if a part has been damaged and requires replacement. One shortcoming of this approach is the possibility of damage occurring resulting in failures between examinations. In addition, test instruments which have been used to inspect bridges are fairly sophisticated, relatively expensive and require operation by skilled personnel.
Strain gages have been used to monitor the stresses to which bridge parts are exposed. They are externally applied and have limited life. Data developed over this limited time is assumed representative of that over an extended period of time. Sometimes certain conclusions about the past are drawn or predictions about the future are made from these inadequate data. The risks of this technique are apparent. There is no assurance that the loads to which the bridge is subjected during the limited periods of monitoring accurately represent the bridge loading over longer periods of service.
As a practical matter, the use of strain gages to monitor the stresses induced in bridge parts is usually restricted to limited periods of time. Strain gages require external sources of power and usually the strain gage unit is powered by a battery. Because batteries become discharged or wear out, they must be replaced at regular intervals. In addition, special care is required to protect the battery against destruction or premature failure of the battery which can result from exposure to the environment. This adds cost to such units. Furthermore, strain gages are typically mounted on exposed surfaces of members where they are subject to vandalism or may be damaged by storms or vehicle accidents.
Still another technique employed in the past to monitor stresses to which bridge parts are exposed has made use of a piezoelectric unit as an acoustic emission detector and measuring device. When a part is subjected to a load, accoustic emission occurs. By analyzing the emission, information about the load may be developed. A serious problem with this technique is that arrays of acoustic transducers with sophisticated instrumentation and extensive wiring are required to determine precisely the source and location of the emission being analyzed. As a result, arrangements of this type are not particularly suitable for monitoring bridge members and similar parts. Also, if surface mounted, they are susceptible to storms, accidents and vandalism.